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In reply to Meltzer's letter, the measurements using the (Calbiochem) CPK Supercalsul were done with water blank and substrate without sera or CSF for controls. One international unit of CPK will catalyse the transfer of 1 μmol of phosphate per minute between creatine phosphate and adenosine diphosphate. Thus Meltzer's statement regarding expression of units in international units per liter or milliunits per milliliter is correct when using the Supercalsul method. The substrate only measures CPK activity; thus other enzymes such as lactic dehydrogenase do not enter the evaluation product. Temperature control and timing is extremely important in the assay. Therefore doubling the amount of CSF and reading the change in optical density of the spectrophotometer at the five minute interval did give us results that seemed reliable. Other time intervals and temperature changes did not give us reproducible results on any one sample. It seems that
KATZ R. CREATINE PHOSPHOKINASE ACTIVITY IN CEREBROSPINAL FLUID-Reply. Am J Dis Child. 1971;122(1):86. doi:10.1001/archpedi.1971.02110010121026